Chris, Vx (V sub x) could be a general reference to voltage, if that was a circuit/electricity problem, or could also have been velocity in the 'x' direction depending on the context. If you're just looking at the sample tests for some of this, we can absolutely help out...just would need to know what you're looking at.
No one has books that give the answers to questions on any of the official ASTB forms currently in use (3, 4, 5). In fact, forms 1 and 2 were retired several years ago after the answers were leaked. However, there are several references published that have sample ASTB tests, and good study materials for the content on the ASTB.
A lot of these resources are maintained by forum member mmx1 here:
http://www.grafitto.com/~mark/astb
Commonly cited study materials are the
"Marine Gouge" along with the
"Mechanical Gouge".
There are two published books that have ASTB study guides and sample tests:
ARCO Military Flight Aptitude Tests (PDF)
Barron's Military Flight Aptitude Tests
These sample tests are generally modeled after the material that is on the current form of the ASTB (with the exception that the actual ASTB no longer contains the Biological Inventory (BI) portion).